Tunable band-pass coaxial filter



Jan. 6, 1953 P. l. RICHARDS TUNABLE BAND-PASS COAXIAL FILTER Filed Jan.3, 194a FIG.2

FIG?) INVENTOR. PAUL I. RICHARDS W w M n N R m T A Patented Jan. 6, 1953TUNABLE BAND-PASS COAXIAL FILTER Paul I. Richards, Cambridge, Mass.,assignor to the United States of America as represented by the Secretaryof War Application January 3, 1946, Serial No. 638,898

3 Claims.

This invention relates to electrical devices and particularly toband-pass filters.

In the use of band-pass filters, particularly at high radio frequencies,it is desirable that the mid-frequency of the pass band be readilyadjustable over a wide frequency range. An object of the presentinvention is to provide a novel band-pass coaxial filter having anadjustable "Id-frequency.

For a better understanding of the invention together with other objects,features and advantages thereof, reference is had to the followingdescription taken in connection with the accompanying' drawing in which:

Fig. 1 is a diagrammatic representation of a coaxial filter of the typecontemplated by the present invention;

Fig. 2 is a schematic illustration of an electrical network analogous tothe arrangement shown in Fig. 1; and

Fig. 3 is a cross-sectional view of a proposed filter assembly embodyingthe invention.

Referring to Fig. l which shows a representative band-pass coaxialfilter as contemplated by the invention, a coaxial transmission lineconsisting of a center conductor I and outer cylindrical conductor I Iis provided with three seriesconnected condensers I2, I3 and I4 and twoshunt stubs I5 and I6. This filter structure is analogous to thelumped-constant band-pass filter shown in Fig. 2, wherein the parallelcombination of an inductance 20 and condenser 2| replaces the shunt stubI5 while the parallel combination of inductance 22 and condenser 23replaces the shunt stub I6 in Fig. 1. Each of the stubs I5 and I6 isone-quarter wave length long at the upper cutofi frequency of the passband and its characteristic impedance is determined by the desired lowercutoff frequency. The capacities of the condensers I2, I3 and I4 aredetermined by the desired characteristic impedance of the filter at aparticular point in the pass band.

Referring now to Fig. 3 which illustrates the construction of a filterunit in accordance with the present invention, the entire filter ismounted in a metal block 25, preferably of brass, which serves as theouter conductor for the various coaxial sections. The block 25 can beformed of two or more pieces to facilitate construction thereof ifdesired. Coaxial connectors 26 and 21 are mounted on the block at theopposite extremities of a hole bored transversely through the block 25.A short metal plug 28, soldered or otherwise secured to the center prongof the connector 26, extends into the bore and terminates in spacedrelation to another short metal plug 29. An insulating sleeve 30 ofdielectric material such as polystyrene encloses the plugs 28 and 29 andmaintains them in the relative position as shown. A third metal plug 3Iis secured to the center prong of the connector 21 and terminates inspaced relation to the plug 29. An insulating sleeve 32 similar to thesleeve 33 encloses the plugs 29 and 3|.

The center conductors of the coaxial stubs as I5 and I6, Fig. 1, areafforded by metal cylinders 35 and 36, Fig. 3, which are positioned insuitable holes bored lengthwise into the block 25 from one end thereofand communicating with the transverse bore through this block. Theconducting bodies 35 and 36 are provided with suitable transverseopenings in which the sleeves 30 and 32 are fitted. This arrangement, ineffect, is equivalent to that of Fig. 1 comprising the three seriescondensers I2, I3, and I4, the respective junctions of which areconnected to the center conductors of the shunt stubs I5 and I6. Theconductors 35 and 36, Fig. 3, are spaced from the surrounding block 25to form the desired concentric transmission line resonators. To enablethe lengths of the concentric transmission line resonators to beadjusted, a pair of tubular metal plungers 31 and 38 is provided. Eachplunger as 31 or 38 comprises a cylindrical shell having a sliding fitbetween the conductor 35 or 36 and the block 25. The plungers 31 and 3Bare mechanically interconnected by a metal strap 39 or the like. Theentire plunger assembly may be moved in or out to adjust the length ofthe concentric transmission resonators. This serves to vary themid-frequency of the filter pass band.

While there has been described what is at present considered to be thepreferred embodiment of this invention, it will be obvious to thoseskilled in the art that various changes and modifications may be madetherein without departing from the scope of the invention.

The invention claimed is:

1. A band-passcoaxial filter comprising a conducting block forming anouter conductor of the coaxial filter, said block having two lengthwisebores and a communicating transverse bore, coaxial line connectorsmounted on said block at opposite ends of said transverse bore, twoconducting cylinders one disposed in each of said lengthwise bores toafford concentric transmission line resonators, each of said cylindershaving a transverse opening axially aligned with the transverse bore insaid block, two metal plugs e.

tending respectively from the center conductors of said connectors intothe openings in said conducting cylinders, a third metal plug extendingbetween said cylinders and terminating in spaced relation to said firsttwo plugs, first and second insulating sleeves respectively enclosingsaid first and third plugs and said second and third plugs, said sleevesbeing fitted within the openings in said cylinders and affordingcapacitive coupling between said two resonators and respectively betweensaid connectors and said resonators, and tubular plungers respectivelydisposed within said lengthwise bores and fitted around said cylindersfor adjusting the effective lengths of said resonators.

2. A band-pass coaxial filter comprising a conducting block forming anouter conductor of the coaxial filter, said block having a lengthwisebore and a communicating transverse bore therein, coaxial lineconnectors mounted on said block at opposite ends of said transversebore, a conducting cylinder disposed in said lengthwise bore to afiord aconcentric transmission line resonator, said cylinder having atransverse opening therein axially aligned with said transverse bore, arigid member consisting of alternately arranged conducting anddielectric pieces disposed in said transverse bore and interconnectingthe center conductors of said connectors, said member passing throughthe opening in said cylinder and serving to retain said cylinder againstmovement thereof, and a tubular plunger disposed in said lengthwise boreand having sliding electrical contact with said cylinder and said blockfor adjusting the eifective length of said resonator.

3. A tunable band pass filter comprising a coaxial structure having aninner conductor comprising a plurality of axially aligned, spaced, metalplugs coupled to each other for electromagnetic energy transfertherebetween, and an outer conductive member spaced from said plugs,input and output coaxial line connectors at each end of said outerconductive member, the end plugs of said plurality of plugs beingrespectively connected to the inner conductors of said connectors andsaid outer conductor to the outer conductors of said connectors, twosections of coaxial transmission line arranged side by side and perpendicularly with respect to the axis of said plurality of plugs, theouter conductors of said coaxial transmission line sections beingelectrically connected to said outer conductive member of said coaxialstructure and the inner conductors of said coaxial transmission linesection both being capacitively coupled to the center plug of saidplurality of metal plugs and each being capacitively coupled todifferent end plugs of said plurality of metal plugs, and mechanicallyinterconnected tuning plungers for simultaneously adlusting theeffective lengths of said coaxial transmission line sections.

PAUL I. RICHARDS.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,196,272 Peterson Apr. 9, 19402,220,922 Trevor Nov. 12, 1940 2,284,529 Mason May 26, 1942

